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How can I find one index for each row of a 2d matrix where a condition is met?

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funkadelala
funkadelala on 13 Feb 2015
Commented: Geoff Hayes on 21 Feb 2015
A is an n by n matrix. B is an n by 1 matrix.
I would like to find the last instance of an element in each row of A being greater than the element in the corresponding row of B, and return an n by 1 list of indices.
For example:
A = [1 3 2;
2 5 -1;
0 2 3]
B = [1;
0;
2]
idx = [3;
2;
3]
I'm sure I could do this with a for loop, but would like to avoid it as the matrices are potentially very large.
Many thanks in advance.

Answers (1)

Geoff Hayes
Geoff Hayes on 13 Feb 2015
Funkadelala - try using the following which assumes that A and B have been defined as above
cell2mat(arrayfun(@(k)find(A(k,:)>B(k),1,'last'),1:size(A,1),'UniformOutput',false)')
We use arrayfun to apply the anonymous function
@(k)find(A(k,:)>B(k),1,'last')
to the kth row of A, using find to find the last index which is greater than the kth element of B. The result from arrayfun is a cell array (row), so we transpose it and convert to a matrix using cell2mat to produce the desired result of
ans =
3
2
3
Try the above and see what happens!
  2 Comments
Geoff Hayes
Geoff Hayes on 21 Feb 2015
funkadelala - are you using a different A and B than the ones described from above, and if so, what are they? Or, what is the output of just
arrayfun(@(k)find(A(k,:)>B(k),1,'last'),1:size(A,1),'UniformOutput',false)

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